Regeneration of fission-products-containing magnesium-thorium alloys



United States Patent Ofi ice 3,120,435 Patented Feb. 4, 1964 3,120,435REGENERATION OF FISSION-PRODUCTS-CON- TAINING MAGNESIUM-THORIUM ALLOYSPremo Chiotti, Arnes, Iowa, assignor to the United States of America asrepresented by the United States Atomic Energy Commission No Drawing.Filed June 4, 1962, Ser. No. 199,998

5 Claims. (Cl. 7584.1)

This invention deals with the regeneration of neutronbombarded magnesiumthorium alloys, and in particular with the removal of fission productstherefrom.

Fission-products-containing magnesium-thorium alloys are obtained in theprocessing of so-called thorium blankets used in nuclear breederreactors, for instance by the process that is the subject matter of US.patent No. 2,990,273, granted to Premo Chiotti on June 27, 1961. In thefirst step of that patented process the neutronbombarded thorium metalis reacted with magnesium at from 800 to 900 C. whereby a liquidmagnesiumthorium-fission products alloy anda solid metal predominantlyconsisting of uranium and protactinium form. The instant process dealswith the regeneration of this magnesium-thorium-fission product alloy.

The fission products usually present in the magnesiumthorium alloy arealkali metals, alkaline earth metals, lanthanide rare earths(hereinafter referred to simply as rare earths), and yttrium. Apart fromfission products, the alloy also contains trace amounts of protactiniumand uranium.

It was found that magnesium chloride is a selective oxidizing agent forthe alloy to be processed, since it chlorinates most of the fissionproducts enumerated above, but does not appreciably chlorinate thorium,uranium or protactinium. This finding is utilized in the process of thisinvention.

The process of this invention broadly comprises adding to the moltenmagnesium-thorium alloy potassium chloride-lithium chloride-magnesiumchloride, whereby the fission products, such as alkali metals, alkalineearth metals, rare earth metals, including samarium and yttrium, and aminor fraction of the thorium, are chlorinated and taken up by a saltphase, while practically all protactinium and minor fraction of calcium,cerium and lanthanum remain in a metal phase; separating the salt andthe metal phases from each other; scrubbing the salt phase with a binaryzinc alloy containing a small amount of magnesium, whereby the chloridesof cerium, yttrium, rare earths (except samarium), uranium and thoriumare reduced to the metals and taken up by a magnesium-zinc phase, whilethe alkali metal chlorides, alkaline earth metal chlorides and samariumchloride, SmCl are retained in a chloride phase; and separating themagnesiumzinc phase from the chloride phase.

The magnesium-thorium alloy is preferably, although not necessarily, theeutectic which contains about 58% by weight of magnesium and melts at582 C.

The mixture of potassium chloride-lithium chloridemagnesium chloridepreferably contains the eutectic of the potassium and lithium chlorides(44.4% LiCl; melting point 354 C.) and magnesium chloride in aconcentration of from to 25% by weight. A quantity of from 400 to 500grams of this salt mixture per one kilogram of magnesium-thorium alloyto be treated was found satisfactory; however, a greater quantity can beused. The operating temperature preferably ranges between 600 and 650 C.The separation of the salt phase from the metal phase can be carried outby customary means, for instance by cooling for solidification of onephase and decantation.

The magnesium-zinc alloy that is added to the salt phase can have amagnesium content of between 2 and 4%, the preferred concentration beingabout 3%. A quantity of between and 250 grams for 500 grams of salt isadequate. The magnesium-zinc phase is separated from the chloride phaseand either discarded, or it is purified by distillation of the zinc andmagnesium away from the fission products, whereby the latter areobtained in a concentrated, compact form ready for disposal. Thechloride phase can be recycled until it has an undesirably highradioactive content of alkali and alkaline earths.

The reduction of the rare earth chlorides with and their extraction intothe magnesium-zinc alloy could not be predicted or expected fromthermodynamical considerations.

The metal phase obtained in the treatment with magnesium chloride andcontaining thorium and the bulk of the protactinium can be treated forrecovery of thorium. One way of accomplishing this is by reaction withhydrogen at between 650 and 675 C. and atmospheric pressure wherebythorium hydride, ThH precipitates from the metal to form a magnesiumsolution containing a remainder of about 8% by weight of thorium. Theprecipitated thorium hydride is separated by customary means and thensubjected to vacuum distillation at about 700 0, whereby both hydrogenand magnesium are volatilized, and a magnesium-free thorium sponge isobtained. This process of hydriding the magnesiumthorium phase per se isnot part of this invention.

In the following an example is given to illustrate the process of thisinvention.

Example To 2000 grams of a magnesium-thorium alloy containing 40% byweight of thorium and also 0.100 each of yttrium, cerium, neodymium andsamarium, 500 grams of a potassium chloride-lithium chloride eutecticcontaining 15% by weight of magnesium chloride are added. The mixture isheated in a tantalum crucible and maintained therein in an argonatmosphere at a temperature of between 600 and 625 C. for four hours;during the entire period the mixture is stirred. The mass is thenallowed to settle for one hour, the salt and metal phases are separatedfrom each other and analyzed. The salt phase contains 0.26% of yttrium,0.15% of cerium, 0.15% of neodymium, 0.4% of samarium and 0.074% ofthorium. The magnesium-thorium metal phase contains 0.035% of yttrium,0.062% of cerium, 0.063% of neodymium and 0.0008% of samarium.

The salt is then transferred to another tantalum container and contactedthere, by stirring, with 250 grams of a binary zinc-magnesium alloycontaining 3% by Weight of magnesium at a temperature of 600 C. Afterfour hours of equilibration and one hour of settling, the salt andzinc-magnesium metal phases are separated from each other and analyzed.The zinc contains, in solution or as precipitated metal, 0.45% ofyttrium, 0.25% of cerium, 0.29% of neodymium, 0.024% of samarium and0.15 of thorium; the salt contains 0.035% of yttrium, 0.0045% of cerium,0.0049% of neodymium, 0.40% of samarium and 0.001% of thorium.

This cycle of treatment with potassium chloride-lithiumchloride-magnesium chloride, phase separation, treatment withzinc-magnesium alloy and subsequent phase separation is repeated twomore times, each time using the same zinc-magnesium alloy, and the samecharge of the chloride mixture. After the three cycles themagnesium-thorium alloy contains 0.0083% of yttrium, 0.029% of cerium,0.033% of neodymium and 0.0008% of samarium, which amounts to a removalof 92, 71, 67 and 99.2% of these respective contaminants in the threecycles. The total amount of thorium that is transferred to thezinc-magnesium alloy is only 1.1 grams.

The zinc-magnesium metal, after the third cycle, contains 0.65% ofyttrium, 0.55% of cerium, 0.52% of neodymium, 0.026% of samarium and0.43% of thorium. The salt obtained after the third equilibration withthe zinc-magnesium alloy contains 0.035% of yttrium, 0.0045% of cerium,0.0049% of neodymium, 0.40% of samarium and 0.001% of thorium; these arethe same contents as those obtained after the first equilibration, whichis due to the fact that the zinc-magnesium alloy becomes saturated inthe first equilibration with respect to yttrium, cerium, neodymium,Samarium and thorium.

These results show that a good decontamination of the magnesium-thoriumalloy from the fission products is obtained by the process of thisinvention.

It will be understood that the invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

What is claimed is:

1. A process of regenerating a magnesium-thorium alloy containingfission products and residual amounts of protactinium and uranium,comprising adding to the molten alloy molten potassium chloride-lithiumchloridemagnesium chloride, whereby the fission products, such as alkalimetals, alkaline earth metals, rare earth metals, yttrium and minorfractions of the thorium and uranium, are chlorinated and taken up by asalt phase, while practically all protactinium and minor fractions ofcalcium, cerium and lanthanum remain in a metal phase; separating thesalt and the metal phases from each other; scrubbing the salt phase witha molten binary zinc-magnesium alloy containing from 2 to 4% by weightof magnesium, whereby the chlorides of cerium, yttrium, trivalent rareearths, uranium and thorium are reduced to the metals and taken up by amagnesium-zinc phase, while the alkali metal chlorides, alkaline earthmetal chlorides and Samarium chloride are retained in a chloride phase;and separating the magnesium-zinc phase from the chloride phase.

2. The process of claim 1 wherein the magnesiumthorium alloy has aboutthe composition of the eutectic.

3. The process of claim 1 wherein the potassium chloride-lithiumchloride-magnesium chloride mixture contains potassium chloride andlithium chloride in about the eutectic composition and magnesiumchloride in a quantity of from 10 to 25% by weight and wherein areaction temperature of between 600 and 650 C. is maintained.

4. The process of claim 3 wherein the magnesium chloride content isabout 15% by weight.

5. The process of claim 1 wherein the magnesium content is about 3% byweight.

References Cited in the file of this patent UNITED STATES PATENTS2,914,399 Dwyer et al. Nov. 24, 1959 2,968,547 Lyon et al Jan. 17, 19612,990,273 Chiotti June 27, 1961 3,053,650 Teitel Sept. 11, 19623,063,830 Martin et al. Nov. 13, 1962 FOREIGN PATENTS 610,220 CanadaDec. 6, 1960

1. A PROCESS OF REGENERATING A MAGNESIUM-THRORIUM ALLOY CONTAININGFISSION PRODUCTS AND RESIDUAL AMOUNTS OF PROTACTINIUM AND URANIUM,COMPRISING ADDING TO THE MOLTEN ALLOY MOLTEN POTASSIUM CHLORIDE-LITHIUMCHLORIDEMAGNESIUM CHLORIDE,WHEREBY THE FISSION PRODUCTS, SUCH AS ALKALIMETALS, ALKALINE EARTH METALS, RARE EARTH METALS, YTTRIUM AND MINORFRACTIONS OF THE THORIUM AND URANIUM, ARE CHLORINATED AND TAKEN UP BY ASALT PHASE, WHILE PRACTICALLY ALL PROTACTINIUM AND MINOR FRACTONS OFCALICUM, CERIUM AND LANTHANUM REMAIN IN A METAL PHASE; SEPARATING THESALT AND THE METAL PHASES FROM EACH OTHER; SCRUBBING THE SALT PHASE WITHA MOLTEN BINARY ZINC-MAGNESIUM ALLOY CONTAINING FROM 2 TO 4% BY WEIGHTOF MAGNESIUM, WHEREBY THE CHLORIDES OF CERIUM, YTTRIUM, TRIVALENT RAREEARTHS, URANIUM AND THORIUM ARE REDUCED TO THE METALS AND TAKEN UP BY AMAGNESIUM-ZINC PHASE, WHILE THE ALKALI METAL CHORIDES, ALKALINE EARTHMETAL CHLORIDES AND SAMARIUM CHLORIDE ARE RETAINED IN A CHLORIDE PHASE;AND SEPARATING THE MAGNESIUM-ZINC PHASE FROM THE CHLORIDE PHASE.